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Up and Down Staircases

Stage: 2 Challenge Level: Challenge Level:1

Up and Down Staircases

One block is needed to make an up-and-down staircase, with one step up and one step down.

single cube

$4$ blocks make an up-and-down staircase with $2$ steps up and $2$ steps down.

four cubes in 2 step up and down staircase

How many blocks would be needed to build an up-and-down staircase with $5$ steps up and $5$ steps down?

Explain how you would work out the number of blocks needed to build a staircase with any number of steps.

Why do this problem?

This problem could be a good way to introduce children to square numbers. It is an engaging, practical context in which to give them the opportunity to explore patterns and to generalise.

Possible approach

You could introduce this problem by building the staircases with one and two steps respectively. Explain how each is named and ask how many blocks are needed to build each. Ask the children to close their eyes and imagine the next staircase, which would have three steps up and three steps down. Can they visualise the number of blocks needed in total? Ask a few learners to explain how they were picturing the staircase in their head and how they knew the total number of blocks.

Set up the challenge of wanting to know the number of blocks in a five-step staircase and encourage pupils to work in pairs. Some may like to use blocks to physically make the staircase, others may want to draw it, or part of it, others may be happy visualising and calculating. Draw the group back together again and share their different methods.

Suggest that you'd like to know how many blocks would be needed to build a much bigger staircase, for example twenty steps up and twenty down. Invite pupils to suggest how they might answer this question if they didn't have enough cubes. Some may say to draw it, but you could protest that this would take too long! Encourage them to look carefully at the numbers they have found so far, perhaps by drawing a table on the board:

Number of steps up Total number of blocks
1 1
2 4
3 9
5 25

Children do not necessarily need to know about squaring numbers in order to express the relationship, it can be explained in terms of "multiplying a number by itself". You may find they need to create more staircases before being able to generalise fully. Once the relationship is articulated, they will enjoy working out the number of cubes needed for huge staircases! As a final challenge, ask them if they can see why square numbers are produced. You may like to show the interactivity in the hints , or use cubes to show the same thing.

Key questions

Can you predict the number of cubes in the next staircase? How did you know?
Do you notice any patterns in the number of steps compared with the total number of cubes?
How could you record your results for each staircase?

Possible extension

This document gives details of two possible extension ideas. Children could explore the numbers of cubes in each 'column' of the staircases, or investigate other kinds of staircases.

Possible support

You could supply some children with a table ready to be filled in.